Fluid Dynamic Studies of Bloody Fingermarks

Fingermark investigations and testimony rely in part on the inspection of minutiae between a known finger and a fingermark, and on the expert’s judgment of the level of agreement or disagreement between the mark and the print. This assessment is heavily dependent on the quality of a fingermark. The quality of a fingermark depends on multiple factors (and their combinations) such as the type of contact, the material and structure of the substrate surface, and the natural residue or contaminant present on the skin and on the substrate surface.

At present, there is limited research and data to help fingerprint examiners assess potential discrepancies between marks and prints when the marks involve blood. The presence of blood at the interface between the finger (the friction ridge skin) and the substrate surface has a complex, non-linear effect on fingermarks. That effect has been experimentally shown to be significant, and is still poorly understood. The proposal will systematically investigate how blood influences fingermarks with controlled experiments in simplified geometries. Experiments will be performed with inverted microscopy coupled with sideways visualization, specifically designed rubber stamps and a novel stamping device. Several variables will be investigated independently:
- Thickness of the blood layer
- Location of the blood (on finger or on substrate)
- Shape of features on the finger
- Finger ridges (size, shape, coating)
- Substrate (material, roughness/texture, coating)
- Timing

A science base will be developed from theory and simulations of fluid dynamics (capillary flow), to explain how ridges (minutiae and other features) are transferred during the formation of bloody fingermarks. The combined experimental and theoretical work will identify conditions in which the following phenomena may occur: tonal inversions (reversals), filling of creases and connective ambiguities for minutiae.

Overall, the study will describe scenarios that could then be empirically tested to see if they apply to forensic situations. A consultant from one of the world’s leading fingerprint team will be involved in the design and the evaluation of the study. Research will be published in forensic science and fluid dynamics journals. The main outcome of this proposal will be that fingermark experts will be provided with quantitative rules to describe how blood interferes between the mark and the capillary ridges.

Note: This project contains a research and/or development component, as defined in applicable law, and complies with Part 200 Uniform Requirements - 2 CFR 200.210(a)(14).

CA/NCF